CHIP28, CHannel-forming Integral membrane Protein is the principal human water channel. CHIP28 is a very hydrophobic protein, and is prevalent in kidney, erythrocytes, colon and a large number of other tissues. It functions as a diffusion-limited passive pore that dissipates osmotic gradients across cell membranes. We seek to understand a molecular basis for water transport by solving the crystal structure of CHIP28.